The present invention relates to a hard coat film comprising a hard coat layer on a plastic base film. More specifically, the present invention relates to a hard coat film with excellent abrasion resistance and superior crack resistance, adhesion and resistance to the effects due to deformation, etc. of the plastic base film. The present invention is suitable as a touch panel surface protective film, for the surfaces of CRT, LCD, PDP and other displays, as well as household electric appliances.
In recent years, plastic products have been replacing glass products because of their workability and light weight. However, since plastic products are more prone to surface damage, they are usually covered with hard coat films in order to provide abrasion resistance. It is also becoming more common to cover conventional glass products with plastic films to prevent scattering when being broken, but because of their lack of hardness, hard coats are often formed over their surfaces.
A conventional hard coat film is usually produced by forming a thin coating film of a thermosetting resin or an ionizing radiation curing resin such as an ultraviolet curing resin, to about 3-15 xcexcm either directly on a plastic base film or via a primer layer of about 1 xcexcm.
However, while such conventional hard coat films have provided sufficient hardness, the fact that the coating films are of low thickness has led to the problem of deformation of the hard coat layers in response to deformation of the underlying plastic base films, resulting in lower overall hardness of the hard coat films, and their performance has therefore been unsatisfactory. For example, when polyethylene terephthalate films, which are widely used as plastic base films, are coated with hard coat films of ultraviolet curing paints to the thickness mentioned above, a pencil hardness of level 3H is common, but this does not approach the pencil hardness of glass which is 9H.
On the other hand, if the thickness of the hard coat layer is simply increased over the usual 3-15 xcexcm, the hardness of the resulting hard coat film is improved but this leads instead to the problem of easier cracking and peeling of the hard coat layer, accompanied by more curling of the hard coat film due to cure shrinkage. For this reason, it has been difficult by conventional techniques to obtain hard coat films with satisfactory properties for practical use.
It is an object of the present invention to overcome the aforementioned problems of hard coat films which employ plastic base films, by providing a hard coat film wherein cracking and peeling of the hard coat layer of the hard coat film is effectively prevented and reduction in the hardness of the hard coat film due to deformation of the plastic base film is suppressed, and which has a pencil hardness in the range of 4H-8H approaching the hardness of glass.
The hard coat film of the invention designed to overcome the problems described above is a hard coat film comprising a plastic base film, a buffer layer composed of one or a plurality of layers with a thickness of 3-50 xcexcm formed on at least one side of the plastic base film, and a hard coat layer with a thickness of 3-15 xcexcm formed on the buffer layer, characterized in that the respective pencil hardnesses of the plastic base film, buffer layer and hard coat layer have increasing values in that order, in such a manner that the hard coat film as a whole has a pencil hardness of 4H to 8H.
Here, the buffer layer has the function of buffering the effects of any deformation of the plastic base film.
In the hard coat film of the present invention having the above-mentioned construction, it is normally preferred for the pencil hardness of the plastic base film to be 4B to HB, the pencil hardness of the hard coat layer to be 3H to 5H and the pencil hardness of the buffer layer to be a pencil hardness intermediate between the pencil hardness of the plastic base film and the pencil hardness of the hard coat layer, for a greater pencil hardness of the hard coat film as a whole.
The material used to form the hard coat layer and/or buffer layer is suitably an ionizing radiation curing resin which can give the desired pencil hardness for each layer and which facilitates film formation on plastic base films.
According to the present invention, the hard coat film may also be provided with glare resistance. Glare resistance may be provided, for example, by forming, irregularities in the surface of the hard coat layer.
According to the present invention the hard coat film may also be provided with an anti-reflection property. An anti-reflection property can be achieved by forming an anti-reflection layer on the surface of the hard coat film.
The xe2x80x9cPencil hardnessxe2x80x9d used in the present invention is the value obtained by the pencil hardness test based on JIS-K5400, and it represents the hardness of the material measured. The pencil hardness test includes five repeated measurements by the pencil hardness test procedure, with the pencil hardness being the hardness of the pencil used for the test when none of the measurements result in scratches or other disturbances to the appearance. For example, if a 3H pencil is used for five test procedures and no appearance disturbances occur, then the pencil hardness of the material is at least 3H.
The pencil hardness can be used to measure not only single layers but also the laminated surfaces of laminated bodies, in the same manner as single layers. In particular, the xe2x80x9clayer pencil hardnessxe2x80x9d according to the present invention is the pencil hardness determined by the same pencil hardness test using a sample of a film formed by a layer forming material on the plastic base film used for the hard coat film, for example a polyethylene terephthalate film of 100 xcexcm or more. In this case, it is preferred for the pencil hardness to be determined for a fixed dry film thickness, such as about 5 xcexcm, of the film formed of the layer forming material.